Comparison of radio frequency circuit losses



v Sept. 21, 1948. V W.VAN B. ROBERTS 2,449,621

COMPARISON OF RADIO FREQUENCY CISCUIT LOSSES Filed'Sept. so, 1942 INVENTOR lfllzrm W/V 6. fiwmrs.

ATTORNEY Patented Sept 2 1, 1948 COMPARISON OF RADIO FREQUENCY CIRCUIT LOSSES Walter van B. Roberts, Princeton, N. J., assignor to Radio Corporation of America, a corporation of Delaware Application September 30, 1942, Serial No. 460,203

9 Claims. (Cl. 175-183) The present invention relates to radio frequency measurements and, more particularly, to a method of and means for determining the relative factor of merit of condensers or inductances.

An object of the present invention is the provision of a novel method of comparing the qualing losses in high frequency tuned circuits or in the component parts thereof.

An extremely important property of a capacl tance or an inductance is its factor of merit, more generally called its Q." It is this factor which determines the sharpness of resonance of a tuned circuit. This factor of merit or Q may be expressed by the equation where R. is the radio frequency resistance and X is the reactance of the element under consideration. An increase of resistance, it will be noted, causes a broadening out and lowering of the peak of the resonance curve. Since the voltage amplitude developed across a circuit at resonance per unit voltage applied therein is numerically equal to the factor of merit or Q of the circuit a convenient way of measuring the factor of merit is to apply a small voltage of resonant frequency to the circuit, and then to detect and measure the stepped up voltage developed thereacross. The ratio of the developed voltage to the impressed voltage is then the factor of merit or Q" of the circuit. However, without utilizing very sensitive meters having arrangements associated therewith for balancing out the idle current of the detector. this arrangement is not capable of detecting small differences between circuits having nearly equal factors of merit.

In accordance with the principles of the present invention, therefore, it is proposed to utilize an infinite impedance detector across the tuned circuit which is to have its factor of merit determined. The detector has, furthermore, an addi- I tional bias applied to its grid from the grid leak of the oscillator supplying radio frequency to the tuned circuit. This bias prevents current flow through the detector until the voltage developed across the circuit exceeds a predetermined value. Thus no balancing current is needed in connection with the meter. Therefore, there is no danger of overloading the meter from the balancing current alone. should fail and thus not supply an additional bias to the detector, the cathode resistance of the infinite impedance detector limits the current to a safe value since. under these circumstances, there is no radio frequency voltage impressed on the detector. These arrangements'also have the advantage that power supply variations cause corresponding variation ln both the radio frequency voltage and said additional bias, so that the ratio of those is little afiected. V

The novel features which, it is believed, are characteristic of the present invention are pointed out with particularity in the appended claims. However, the invention will be more completely understood by reference to the following detailed description which is accompanied by a drawing in which Figure 1 illustrates a somewhat conventional means for determining the factor of merit of a circuit, while Figure 2 iilustrates the application ofthe principles of the present invention to the circuit of Figure 1, and Figure 3 illustrates a modification of the embodiment shown in Figure 2.

In Figure 1 is shown a radio frequency oscillator including a thermionic discharge tube 10 having its anode I I connected to a source of anode supply, as indicated by the arrow labeled B+. The grid I2 is connected to one end of inductance M of tank circuit T1, the other endof the inductance being grounded. In order to generate oscillations, cathode l5 of the tube I0 is connected to a tap on the coil i l of tank circuit T1. The tank circuit is tuned to the desired operating frequency by means of variable condenser IS. A

-grid leak I1 and a grid condenser I8 are connected in circuit between grid l2 and coil M in a.

manner well known in the art. A second resonant circuit T2 having a coil 24 and a variable tuning condenser 28 is inductively coupled to the tank circuit/T1 by means of coupling link 20. Thus a small radio frequency voltage developed by the oscillator in the tank circuitTi may be applied to the tank circuit T2. The radio frequency volt age developed across tank circuit T2 is measured by means of detector 25 and outputmeter M. Assuming that the output meter indicates the Furthermore, if the oscillatoractual radio frequency voltage across T2, and that the applied voltage is known, the factor of merit or Q" of the circuit is the ratio of the output voltage to the voltage impressed on the circuit from the oscillator. As before mentioned, such an arrangement may be satisfactory for the actual measurement of the Q of a circuit but it is not sufficiently sensitive to small changes therein for precise comparison of two circuits having very nearly equal values of Q." The arrangement may be made more precise by utilizing a more sensitive meter at M if a balancin circuit arrangement is provided so that only the change in the developed voltage is indicated by the meter M. However, such balancing current requires careful adjustments and if not properly manipulated may burn out the sensitive meter.

In the arrangement of Figure 2 is shown a modification of the circuit shown in Figure 1 wherein the disadvantages of circuit in Figure 1 are overcome. A very sensitive meter may be employed without endangering the meter through careless adjustment of any balancing current. The oscillator circuit of Figure 2, by means of which a radio frequency voltage is developed in tank circuit T1, is the same as that shown in Figure'l and will not be again separately described. The radio frequency voltage developed in tank circuit T1 is induced into tank circuit T: by means of an adjustable link circuit 36. Tank circuit '1: of Figure 2 is provided with a first capacity 26 and an associated adjustable trimmer condenser 26, a second capacity 36 and an associated trimmer condenser 36', either of which combinations may be selectively inserted into the circuit by means of switch 3i. The radio frequency volta e dea44aea1 sample switched into the circuit by means of condenser 26' and the switch 3|, the trimmer coupling of the link circuit 36 are so adjusted that the resonant voltage across tank circuit T: produces a relatively large deflection of meter M, for example, /2 or /4 of its full scale. Without altering the link coupling 66, switch 3| is then switched to its other position thus connecting condenser 36 across 34, Condenser 36 may have another dielectric sample between its plates.

Trimmer condenser 36' is then adjusted for resonance. The switch 3! may then be thrown back and forth to assure that the change in meter reading is the same each time the switch is opveloped across tank circuit T2 is impressed on grid 39 of a detector tube 46 which is somewhat similar to the so-called infinite impedance detector, already known in the art, in that the anode 4| is directly connected to a source of anode potential, as indicated by the arrow labeled 3+, while the load circuit, including meter M and adjustable resistance 42, is connected in series between cathode 43 and ground. By-pass condensers l3 may be provided from the anodes of tubes 10 and 46 to ground and from the cathode 43 of tube to ground in a conventional manner. The grid 39 of detector tube 46 is isolated from ground by blocking condenser 44 and the gridis supplied with a bias developed across the oscillator grid leak ll by way of the very high resistance 46. This resistor may be connected to an intermediate point on grid resistance ll if it is desired to employ looser couplings at link 36.

It will be seen that unless the voltage developed across tank T2 reaches a certain magnitude the large bias on grid 39 assures that no current will flow through the sensitive meter M so that no balancing current is required in connection with the meter. There is, therefore, no danger of the meter being overloaded by the balancing current itself. Furthermore, as a result of self bias provided by resistor 42 and/or by the resistance of the meter itself, if the oscillator should fail to oscillate and thus fail to provide any grid bias for the detector tube 40, the current through the meter is limited by said self biasing effect to a small safe value. I

In utilizing the circuit of Figure 2 for comparis as follows: With one of the samples inserted between the plates of condenser 26, and with this erated. The change in meter reading indicates the difference between the circuit "62 with one sample in place and the circuit "Q" with theother sample in place. If desired, of course, one of the condensers 26 or 36 may be left empty, in which case the comparison is between a given sample and an equivalent volume of air dielectric. It is assumedxof course, that the condensers 26 and 36 and their associated trimmer condensers 26' and 36' are physically similar and symmetrically arranged so that there is no difference in meter reading when there is no dielectric in either of the holders. 7

Due to the sensitivity of the circuit arrangement described, if there is any great difference in the dielectric quality of the samples placed between the plates of condensers 26 and 36, the meter M may go off scale in one position of switch ill and fail to be deflected at all in the other position of switch3l. In this case, the sensitivity of the instrument may be reduced by cutting in more resistance by means of variable resistance 42 thus, in effect, reducing the change in meter reading between the two positions of switch 3!.

If it is desired to compare the factor of merit of two coils the arrangement of Figure 3 may be used. The arrangement of the oscillator circuit is identical with that of Figures l and 2 and the arrangement of the detector tube 46 is the same as Figure 2. They will, therefore, not be again described.

However, in this case switch 3| is arranged to selectively connect into the circuit coils 34 and 36. Coils 34 and 36 are arranged to be tuned to the frequency generated by the oscillator tube l6 by means of variable condensers 26 and 66. Furthermore, instead of link coupling 36, a capacity coupling arrangement is provided which utilizes two extremely small condensers 6| and 62 connected in series between the high voltage ends of the two resonant circuits, together witha relatively large variable capacity 64 connected between the junction of the two small capacities and ground. The serially connected small capacities 6| and 62 are so chosen as to give somewhat more than ample coupling in the absence of the variable capacity 64. The maximum value of condenser 64 is sufliciently large to reduce the effective coupling.

connected to the detector circuit to as smalla value as required.

It will be noted that the grid bias supply through resistor 46 plays the part of a bucking out current for the meter in that the meter measures only the excess of. voltage above a certain value while the infinite impedance type of detector circuit safeguards the meter against failure of the bias derived from the oscillator. Since the bias .derived'from the oscillator is proportional to the'voltage developed by the oscillater, it has been found in experimental use of the arrangement of Figure 2 that, the system .may be operated without substantial change of any of its adjustments over a wide range of voltage being characterized in that in operation a negative potential with respect to said refer-*- ence point is developed and means for applying a plate voltages where both plates ii and H are 5 energized from the same voltage and source, predetermined portion of said negative potential the only difference noted being increased sensi- 'to the grid of said tube. tivity in the case of higher plate voltage. 2. In combination, a thermionic discharge In ,the above mentioned experimental artube oscillator circuit including a tuned circuit rangement a satisfactory value of resistor 46 and a grid leak, a second tuned circuit, a link was found to be of the order of 5 toil) megohms, circuit coupling said tuned circuits, a second when the oscillator grid leak had a value of thermionic discharge tube having a control about 50,000 ohms, the plate voltage being anyelectrode coupled to said second tuned circuit. thing from 45 to 90 volts. The meter M, having an anode and a cathode, an anode-cathode ciran internal resistance of 10,000 ohms, gave a cult and a control electrode-cathode circuit confull scale deflection for 400 microamperes, while nected thereto, a portion of said circuits being variable resistor 42 had a maximum value of common, a current measuring instrument in the 50,000 ohms. In the experimental arrangecommon portion of said circuits, said control ment the trimmer condensers 26' and 36' of Figelectrode-cathode circuit including a high reure 2 were made as small as possible while still sistance connection from said control grid to resonating the tank circuit T2 with the dielecsaid grid leak, means for selectively connecting a trio sample in place and the oscillator frequency number of condensers in said second tuned cirwas adjusted to bring this about. The tank cult, said condensers each being adapted to incircuit coils used were of 'a type manufactured clude a different dielectric between electrodes for use in a '75 meter amateur band transmitter. thereofa As pointed out previously, the present inven- 3. Means for comparing. circuit losses in 9. tion has its chief usefulness in comparing losses number of resonant circuits including means for in nearly equal circuit components. However, introducing selectively into said circuits a radio an actual measurement of the Q of a circuit frequency voltage of the frequency to which may be made by calibrating the arrangement said circuits are resonant, means for derivinga suitably. For example, if a tuned circuit of direct potential proportional to said radio freknown "62 is substituted for condenser 26" and quency voltages, means for rectifying solely the coil 34 and if condenser 50 is then adjusted so voltage developed across said circuits which exthat at resonance the meter M shows some arceeds in amplitude the amplitudeof'said direct bitrary small deflection, then this known Q" 35 potential and means for determining the magnivalue may be marked on the dial of condenserfi' i. tude of said rectified voltages. g In similar fashion other calibration points may In i ati n a ra io frequency oscillator be marked on this dial. With the dial so calicircuit including a resistance across which a brated, the Q of other circuits of simllar.redirect Dutentiel is e pe o an a plitude actance may be 'readfrom the dial when it is 40 proportional to e a p tude of oscillations adjusted to produce the aforesaid resonance generated by said circuit, a detector-circuit havindication at M and the system is otherwise s a c n o e tr de it and an ou put operated under the same conditions as prevailed r it, m ns for applying said direct potential during calibration. to said control electrode circuit, a current meas- While I have illustrated several modifications urine ns u nt n sa d utput ir t. a resoof the present invention and given certain nant circuit adapted to be tuned to the frespecific values for the circuit components used quency of oscillations generated by said oscillator it should be clearly understood that the present circuit, means for coupling said resonant circuit invention is not limited thereto since many to said oscillator circuit and means for coupling modifications may be made in the several elesaid control electrode circuit to said resonant ments employed in their arrangement and it is, circuit. therefore, contemplated by the appended claims n 8- yste for (tempering Circuit ss to cover any such modifications as fall within means for generating aradio frequency .voltage, the sphere and scope of the present invention. means for applying predetermined amount oi I claim: 7 s said voltage to'a tuned circuit including an in- 1. In a system for comparing circuit losses, ductance and a plurality of condensers adapted means for generating a radio frequency voltage, to be selectively connected across said inductmean for applying a predetermined amount of ance, said condensers having different dielectric said voltage to a tuned circuit including an inmaterials between their electrodes, means for so ductance and a. plurality of condensers adapted 6Q adjusting each of said condensers that they eachto be selectively connected across said inducttune said inductance to the frequency of said ance, said condensers having different dielectric generating means, control electrode, anode and materials between their electrodes, means for so cathode connections for a thermionic tube, adjusting each of said condensers that they each means for connecting a terminal of a source of tune said inductance to the frequency of 'said' anode potentialto said anode connection, an-- generating means, a thermionic discharge tube other terminal of said source being connected having an allude, cathode and grid, means for to a point of zero reference potential, a current connecting the positive terminal oi' a source of responsive instrument connected in. series beanode potential to said anode, the negative tertween said cathode connection and said point minal of said source being connected to a point of zero reference potential, means for connectof reference potential, a connection from said ing said grid connection to one end of said tuned cathode to said point of reference potential, said circuit, the other end of said tuned circuit connection including a current responsive inbeing connected to said point of saidzero referstrument, means for coupling said grid to one ence potential, means for developing a negative end of said tuned circuit, the other end being potential having an amplitude proportional to the amplitude of the voltage generated by said radio frequency generating meansand means for applying a portion of said negative potential to said grid connection.

6. In combination a thermionic discharge tube oscillator circuit including a tuned circuit and a grid leak, a second tuned circuit, a circuit coupling said tuned circuits, 9. second thermionic discharge tube having a control electrode coupled to said second tuned circuit. an anode and a cathode. an anode-cathode circuit and a control electrode-cathode circuit connected thereto, a portion 'of said circuits being common, a current measuring instrument in the common portion of said circuits. said control electrodecathode circuit including a high resistance connection from said control grid to said grid leak, said second tuned circuit including parallel connected inductance and capacitance elements and means forselectively substituting said elements of said second tuned circuit.

7. In combination a thermionic discharge tube oscillator circuit including a tuned circuit and a grid leak. a second tuned. circuit, a circuit coupling said circuits, means for coupling a control grid circuit of a second thermionic discharge tube havlngan anode-cathode path and a control grid to said second tuned circuit, a current measuring instrument connected in series with said anode-cathode path, a high resistance con- 8 of oscillations generated by said oscillator circult, means for coupling said resonant circuit to said oscillator circuit and means for coupling said control electrode circuit to said resonant circuit, said resonant circuit including parallel connected inductance and capacitance elements and means for selectively substituting elements in said resonant circuit.

9. In combination a radio frequency oscillator circuit including a resistance across which a direct potential is developed of, an amplitude nection from said control grid to said grid leak and means for selectively substituting diflerent reactance elements in said second tuned circuit.

8. In combination a radio frequency oscillator circuit including a resistance across which a direct potential is developed of an amplitude proportional to the amplitude of oscillations generated by said circuit, a detector circuit having a control electrode circuit and an output cir' cuit, means for applying said direct potential to said control electrode circuit, a current measuring instrument in said output circuit, a resonant circuit adapted to be tuned to the frequency proportional to the amplitude of oscillations generated by said circuit, a detector circuit having a control electrode circuit and an output circuit, means for applying said direct potential to said control electrode circuit, a current measuring instrument in said output circuit, a resonant circuit adapted to be tuned to the frequency of oscillations generated by said oscillator circuit, means for coupling said resonant circuit to said oscillator circuit and means for coupling said control electrode circuit to said resonant circuit, and means for selectively substituting other similar resonant circuits for said first mentioned resonant circuit;

WALTER. vm B. ROBERTS.

REFERENCES CITED The following references are of record in the file of this patent: 1

UNITED STATES PATENTS 

